A variety of rechargeable, high energy density electrochemical cells have been demonstrated although the most widely utilized commercial system is that based upon Li-ion chemistry because it displays very high energy density. Such cells usually consist of a transition metal oxide or chalcogenide cathode-active material, an anode-active lithium metal or lithium intercalation or alloy compound such as graphitic carbon, tin and silicon, and an electrolytic solution containing a dissolved lithium-based salt in an aprotic organic or inorganic solvent or polymer. Today there is great demand for energy storage devices capable of storing more Wh/l or Wh/kg than premier rechargeable Li-ion batteries are capable of delivering. Consequently an increasingly sought after route to meeting this demand higher energy density is to replace the monovalent cation lithium (Li+) with divalent magnesium cations (Mg2+) because magnesium can enable nearly twice the charge of Li+ to be transferred, per weight or volume. Furthermore the abundance of Mg metal and readily available compounds containing Mg will enable significant cost reduction relative to Li-ion batteries, and it encompass superior safety and waste disposal characteristics.
Electrolytes utilizing alkali metal with organic ligands from organometallic species have been described; however the use of an alkaline earth metal anode such as magnesium would appear disadvantageous relative to the use of an alkali metal such as lithium because alkali metal anodes are much more readily ionized than are alkaline earth metal anodes. In addition, on recharge the cell must be capable of re-depositing the anode metal that was dissolved during discharge, in a relatively pure state, and without the formation of deposits on the electrodes. Furthermore, there are numerous other disadvantages to alkali batteries. Alkali metals, and lithium in particular, are expensive. Alkali metals are highly reactive. Alkali metals are also highly flammable, and fire due to the reaction of alkali metals with oxygen, water or other active materials is extremely difficult to extinguish. As a result, the use of alkali metals requires specialized facilities, such as dry rooms, specialized equipment and specialized procedures. In contrast, magnesium metal and its respective inorganic salts are easy to process and usually considered as benign. The metals are reactive, but undergo rapid passivation of the surface, such that the metals are highly stable. Magnesium is inexpensive relative to the alkali metals, and widely used as ubiquitous construction materials.
Electrolytes that enable reversible, electrochemical deposition of Mg and have potential use in a battery contain organometallic bonds. Most often these electrolytes contain organometallic Grignard salts as the electrochemically active component. However sustaining >1 volt is problematic or impossible with the usual intercalation cathodes because of electrolyte decomposition and corresponding encrustation/passivation of electrode surfaces. Enhanced electrochemical stability has been demonstrated by complexing with strong Lewis acids. For example, a cell comprised of a Magnesium metal anode, a Molybdenum Sulfide “Chevrel” phase active material cathode, and an electrolyte solution derived from an organometallic complex containing Mg is capable of the reversible, electrochemical plating of Magnesium metal from solutions with about a 2 V stability window. Under the same principle similar results have also been shown when you complex inorganic MgCl2 and organometallic Aluminum Lewis Acids.
Such cells are low energy density due to a low voltage reaction between a Chevrel cathode and Mg metal anode and therefore not commercially viable. Sustaining a voltage >2 volts is problematic or impossible with the usual intercalation cathodes and electrolytes based upon Grignard and other organometallic species. Magnesium batteries operating at >1.5 volts are particularly prone to electrolyte decomposition and to encrustation/passivation of the electrode surface. Furthermore electrolytes targeted for use in electrochemical cells wherein the plating and stripping of Mg-ions is required specify organometallic species among the ionic species in the respective electrolytic solutions. There are many disadvantages to organometallic species, relative to inorganic salts. Practically, all organometallic species of the alkalis and the earth alkalis are highly unstable in the presence of air and water and thus classified as pyrophoric. Organometallic species are quite expensive to produce while maintaining purity. Organometallic species introduce organic ligands to the electrolytic solution, which inherently limits the electrochemical stability of the solution. Organometallic species introduce organic ligands to the electrolytic solution, which will limit the chemical stability of the solution when in contact with certain electrode active materials and other electrochemical cell components. All handling and manipulations of organometallic species of this sort are complicated, hazardous and expensive.
Consequently there is a widely recognized need for, and it would be highly advantageous to have, an improved non-aqueous electrolyte that allows the production of a practical, rechargeable magnesium battery which would be safer and cleaner, and more durable, efficient and economical than heretofore known.